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Scientists have postulated the Earth Tectonics hypothesis whose basic assumptions are given below:

(1) Concept of Plates - The whole earth's surface is composed of six plates. These plates are thin but rigid. Their thickness varies from 100-150 km. Their sizes are not equal. Their names are (i) Eurasian (ii) Indian (iii) Pacific Ocean (iv) American (v) African and (vi) Antarctic.

(2) Movement of Plates - These plates are comparatively mobile and their movement is measured with reference to the axis of the earth. (3) Boundaries of Plates - Volcanic, earthquake, tectonic and other activities take place in areas between these plates. These areas are known as zones.

(4) Ocean Ridges - The earth's crust is spreading over ocean ridges. New crust is created. The submerged ridge in the Atlantic Ocean is worth mentioning. There is another important ridge submerged in southern Pacific Ocean. The Eurasian plate situated to the east of them and American plate to the west are receding away from each other but their contact with ocean ridges does not break because new sediment formed keeps them in contact. The rate of recession of plates is one cm. per year near Iceland but it is 9 cm. near Equator in Atlantic Ocean. This type of plate development is called 'constructive'.

When the plate development brings them closer to one another, the development is known as 'destructive'. This shortens the area between the plates. The coming nearer or going away of the plates balances each other so that the area of the earth's surface remains almost constant.

Formation of Mountains - When the plates come towards each other, the sediment in the zone area is folded into mountains. This hypothesis is mathematical in nature.

Next: Economic Importance of Plateaus